The response of petioles of Glechoma hederacea to a dynamic light climate

In herbaceous vegetation patterns of light distribution may change over tim e. Prostrate plants growing in such a dynamic light environment may benefit from petioles that respond plastically to changing light conditions. In an experiment, the response of petioles of Glechoma hederacea to changing lig ht conditions was analyzed. Treatments included continuous shade, continuou s high light, a shift from shade to high light and from high light to shade when the plants had formed 10 ramets. In all four treatments, even petiole s that had apparently ceased growing, were still able to elongate slightly but the extent of elongation decreased with the age of the petiole. In the oldest petioles relative extension rates were higher in shade than in high light. In plants that were exposed to full daylight in the second half of t he experiment, even newly formed petioles were longer than those in plants that grew in full daylight continuously though they had elongated over a sh orter period. In plants that were shaded in the second half of the experime nt, only the youngest 4 to 5 petioles reached lengths similar to that in co ntinuous shade. This mechanism may enable plants to keep young (productive) leaves in the upper layers of the canopy while other less productive leave s remain at lower levels of the vegetation.